It is prior art to perform surgical operations for correction of ametropia or other therapeutical treatments (e.g. cuts for ceratoplastics) by means of laser radiation or other radiation interacting with parts of the eye in order to change the optical properties of the eye in a selected manner. Most prominent example of the interaction between laser radiation and the eye is the re-shaping of the cornea by ablation (removal of tissue). The present state of the art typically uses the well-known LASIK technique. The present invention is, in particular, concerned with LASIK technique, however, the invention is generally concerned with ophthalmological surgery with radiation, are in particular laser radiation. Sources for laser radiation, modified in particular UV-sources (e.g. excimer-lasers), IR-sources (e.g. erbium:YAG laser) as well as ultrashort laser pulses (e.g. titan:saphire, Cr:LiSAF, Nd:YLF).
It is also prior art to implant an artificial eye lens (intraocular lens) into the eye by means of a surgical action in order to correct the eye optically. Certain intraocular lens types can be modified after implantation by action of light (e.g. UV-radiation) in order to change the form or the optical effect of the lens. It is also known that certain materials exposed to high light intensities (e.g. ultra-short laser pulses) change its refractive properties, i.e. its index of refraction.
In the prior art the changes of form or certain optical structures of the eye, e.g. the cornea, are calculated on the basis of clinical data (e.g. data regarding ametropia) and corresponding theoretical models (eye models). From the difference between a preoperative form and a theoretical postoperative form of the eye structure to be changed, the so-called treatment profiles or working profiles can be derived and according to such profiles the laser radiation is controlled in space and time. This is all well-known to a person skilled in the art. For example, a preoperative cornea curvature minus a wanted postoperative cornea curvature results in a volume of cornea tissue that is ablated in order to obtain a change in the form of the cornea such that the wanted optical change of the optical system of the entire eye is achieved.
If pulsed laser radiation is used in the so-called spot-scanning-method, i.e. single laser pulses are focused such that its diameters are small as compared to the extension of the cornea and are successively scanned over the cornea, the prior art teaches to generate a complete control program for laser pulses in space and time on the basis of certain assumptions regarding the tissue removal effected by each laser pulse. A list of x, y, z-positions of the laser pulses is derived and, according to that list, the ablation of the cornea surface or the interaction between the laser radiation and the eye tissue is performed. The latter is, at present, mostly performed by ultra-short laser pulses.
It is also known in the prior art, to consider certain factors which have an effect on the result of the surgical action when generating the list of single laser pulses. Such influential factors can be the healing process, biomechanical changes by the intervention itself, smoothing effects in the tissue as well as a tear film.
Such factors to be reckoned on are typically determined empirically on the basis of clinical data and by means of certain evaluation functions, e.g. the point-spread function, modulation transfer function etc. The observation and quantitative calculation of such factors is known to a person skilled in the art.
In the prior art the positions of the individual laser pulses are calculated with reference to a reference axis to be determined. Very often the “line of sight” is selected as the reference axis. This reference axis is then used to control the laser systems. As is known, suitable means for forming and guiding the beam are used in order to shape the individual laser pulses and position them in space. The positioning is performed in three dimensions wherein the x, y, z-coordinates are typically used such that the x, y-plane is perpendicular to the line of sight, whereas the z-axis is in that line. The z-coordinate, therefore, is related to the focusing of the laser beam.
According to EP 1 327 948 A2 a list of laser pulses for ablation is generated by determining for a patient on the basis of aberration measurements a correction list. Thereafter, a databank is used in which surgical results obtained with other patients under similar conditions are stored. On the basis of such data a correction of the initial list is performed. This results in the eventually used ablation profile.
US 2003/0225399 A1 teaches to simulate an alignment error. An iteration algorithm is used to measure an actually ablated surface. That measurement serves in the iteration loops as a comparative value.
WO 02/07660 A2 describes a method for determining an ablation profile under consideration of several parameters, however, there is no combination of simulating, judging and generating iteratively on the basis of negative judgments.
It is known in the prior art (WO 01/85075A1) to take account of reflection losses caused by different angles of incidence upon the cornea when determining an ablation profile. This prior art also teaches to take account of the effect on the fluence caused by different angles of incidence between the radiation and the cornea surface. It is also known to consider the postoperative healing process.
All these known techniques can be improved as they are regularly not specific with respect to the individual patient but rather are based on average values of postoperative clinical results of a large number of patients.
It is known in the prior art to perform refractive surgery wavefront-guided. This results in a remarkable improvement of the results, also at ametropia at higher orders. Nevertheless, such prior art can be improved as the calculation of the ablation profile is based on assumptions and simplifications which limit the precision of the laser interaction.
Prior art according to DE 100 14 480 (Hohla et al.) is based on the observation that a wavefront measurement provides information about the eye, in particular the cornea, in a range within the pupil opening only and that, furthermore, a topography measurement measures also areas of the cornea outside the pupil opening. Accordingly, Hohla combines both measurements. Also a simulation is described how the calculated theoretical ablation profile is subtracted from the preoperative topography in order to obtain the postoperative (wanted) cornea shape. This calculated cornea shape is presented to the doctor on a screen. The doctor shall examine the remaining thickness of the cornea and then decide whether or not the treatment is performed. The simulation has the only purpose to provide a presentation to the doctor as a support to make a decision.